Sensors with implantable or insertable electrode systems facilitate measurements to be made of physiologically significant analytes such as, for example, lactate or glucose in a patient's body tissue. The working electrodes of systems of this type have electrically conductive enzyme layers in which enzyme molecules are bound which release charge carriers by catalytic conversion of analyte molecules. In the process, an electrical current is generated as measuring signal whose amplitude correlates to the analyte concentration.
The electrical conductivity of good enzyme layers should be as high as possible to allow charge carriers that are released to be detected as measuring signal as completely as possible, they should be sufficiently water-permeable to allow analyte molecules to diffuse from aqueous body fluid, usually this will be interstitial fluid or blood, into the enzyme layer, and finally they should bind enzyme molecules contained therein as completely as possible such that these cannot leak into surrounding body tissue.
Suitable enzyme layers can be made, for example, of platinum black, which can be impregnated with enzyme solution and shows good water permeability because of its sponge-like structure, or from electrically conductive particles, for example carbon or metal particles, and a binding agent. Enzyme layers of this type are usually brittle. For this reason, the enzyme layer of the working electrode of known sensors covers only a very small area, usually only a fraction of a square millimeter. An example of this is the electrode system known from U.S. Pat. No. 4,655,880, in which the conductor of the working electrode extends over only approximately 200 μm. In order to increase the local current density, this conductor is provided with an electrically insulating coating into which small openings with a diameter of approximately 10 μm were etched before the conductor was coated over its entire length with an enzyme-containing paste in order to form an enzyme layer.
However, despite extensive research and development, known electrode systems are susceptible to interference and associated with a disadvantage in that they can be used to determine the analyte concentration only at lower accuracy and reliability than is feasible using a conventional ex-vivo analysis.
In order to increase the measuring accuracy, US Patent Application Publication No. 2005/0059871 proposes to measure the analyte concentration of interest using multiple working electrodes simultaneously and to statistically analyze the measurements thus obtained. As an additional measure, it is proposed to use further sensors to determine other analyte concentrations or physiological parameters and test the plausibility of individual results based on the concentration of different analytes thus obtained.
However, the use of a large number of working electrodes not only increases the utilization of equipment resources, but also leads to a problem if inconsistent measuring signals of the individual working electrodes make it unclear which of the different measuring values accurately reflect the analyte concentration in the body of the patient.